This invention relates to a novel curable urethane composition. More particularly, this invention relates to a novel solventless urethane composition having a sufficiently low viscosity to be easily handled and forming a polyurethane resin with excellent elongation and electrical properties.
Use of castor oil as a polyol in the preparation of polyurethane has well been known.
Castor oil is a triglyceride and its fatty acid composition consists of approximately 90% by weight of ricinoleic acid and 10% by weight of non-hydroxyl-containing fatty acid.
Therefore castor oil is a mixture of 70% by weight of triricinolein and 30% by weight of diricinolein. Then, it has a functionality of 2.7 as a polyol. Moreover, the castor oil backbone has a large hydrocarbon sidechain, which tends to force the hydroxyl groups apart. That chemical structure makes castor oil a better urethane material than the well known polyesterpolyols and polyetherpolyols especially as long as electrical property and water resistance are concerned.
Therefore, use of castor oil as a polyol in the preparation of polyurethane is widespread, especially in the electrical industries.
Recently, however, the trend toward minimization of electronic elements has come to require a more flexible resin. And the requirements to be met by electrical insulating materials with regard to mechanical strength, electrical property and temperature resistance have been increased considerably.
In order to get a more flexible polyurethane resin various kinds of means have been proposed and employed.
T. C. Patton et al. (J.A.O.C.S. 36, 149(1959)) have reported various ricinoleic acid polyols for urethane foams and coatings. Some of them, which have less functionality than castor oil, give more elastic polyurethane resins than castor oil does.
In the same manner, Japanese Un-examined Patent Publication No. 85819/1982 (CA: 97 (20) 164709) discloses a urethane potting system for electronic devices, which comprises a mixture of castor oil and ethylene glycol esters of ricinoleic acid. Also, use of plasticizers such as dialkyl phthalates and other oils has been proposed. U.S. Pat. No. 3,714,110 discloses oil-extended urethane systems.
Although these systems possess desired low viscosity, they still have defects. The system comprising glycol esters of ricinoleate exhibits a rather brittle property at low temperature. The oil-extended system is characterized by excessive swelling under conditions of moderately high temperature use, for instance, in encapsulating compositions.
U.S. Pat. No. 4,348,307 discloses another castor oil based urethane system containing monofunctional alcohol, 2-octyldodecanol, which reduces the average hydroxyl functionality of the urethane system and serves to make the cured resin more flexible. This system, however, is rather expensive and it is difficult to control the average hydroxyl functionality and the hardness of the cured resin.
Recently, hydroxyl-containing homopolymers of dienes, hydroxyl-containing copolymers of dienes and/or their hydrogenated derivatives have been employed as a urethane component. Among them, polybutadiene polyols are most commonly used and form flexible polyurethane resins which exhibit a desired elongation even at low temperature.
Polybutadiene based polyols have higher molecular weights and less hydroxyl functionality than castor oil, and therefore form more flexible polyurethanes than castor oil does.
Polybutadiene based polyols, however, have generally too high of a viscosity to be used in a solventless urethane system.
Additionally, polybutadiene based polyols have poor compatibility with well known polyols as polyether polyols, polyester polyols and low molecular weight diols and triols known as chain extenders.
U.S. Pat. No. 4,008,197 and U.S. Pat. No. 4,166,258 disclose mineral oil extended polyurethane systems comprising castor oil and polybutadiene based polyols. Although better compatibility of mineral oil with a polyurethane can be obtained by using a certain coupling agent in those systems, the cured resin obtained exhibits poor stability at relatively high temperature, including weight loss and hardness change.
Japanese Un-examined Patent Publication No. 93717/1983 discloses a urethane composition comprising castor oil, polybutadiene based polyols and plasticizers. But limited compatibility of castor oil with polybutadiene polyols restricts its formulations.
U.S. Pat. No. 4,313,858 discloses a thermally-stable polyurethane system, comprising a polyisocyanate prepolymer, a polyol having pendant vinyl group, a drying oil and a perioxide catalyst. In this system, the drying oil, serving to improve the viscosity and handling characteristics of the system, would react with a double bond of the urethane prepolymer by the aid of a preoxide component, which is cited. Although the Patent system possesses desired low viscosity and would form a thermally-stable polyurethane encapsulating resin, such a system is characterized by a tedious curation of double bond oxidation and a formation of heterogeneous polymer compositions caused by two ways of curation.
An object of this invention is to provide a novel solventless urethane composition, which is useful as casting compounds, encapsulating compounds for electric devices, conformal coatings for printed circuit boards and sealing material for electrical and optical communication cables and cable connectors.
Another object of this invention is to provide a novel urethane composition having the processing advantages typical of urethane systems of desirable low viscosity characteristics.
Further object of this invention is to provide a urethane composition which forms a polyurethane resin of good flexibility and electrical property to meet current requirements.
These and other objects of this invention will become apparent in the detailed descriptions which follow.